Vertices deforming in the wrong way... can't figure it out
Sometimes you can fix this kind of problems with weight paint, maybe it's possible in your case?

For example I've removed the back of the belt from all vertex groups and parent it to the bone called Spine and it seems to work. It won't give you a big control on it though:



You could have also created a different mesh for this part of your character (which would have made sense as it's a differente object than the shirt or pants).

But if we keep your current mesh and want to keep a precise control, for this kind of cases it may better to create some additional bones that will be the only ones to control the part of the mesh with problems, so here for example you could:


Create 3 additional bones that you parent to the spine:





Call them for example backbelt1, backbelt2 and backbelt3:





Select the back of the belt and remove it from all vertex group so that no bone have anymore influence on it:





Create 3 new vertex groups called backbelt1, backbelt2 and backbelt3 and assign each part of the back of the belt to each group. Now the back of the belt is automatically controlled by a bone:





If you bend the character, move the bones so that the belt is not deformed a way you would not want: